Electronic gain control for photomultiplier used in gamma camera

ABSTRACT

An electronic gain control is disclosed for the photomultipliers of a gamma camera which assures that all photomultipliers in the camera have uniform gain for any given ganuna event. A specific dynode in the photomultiplier is isolated from the line resistive voltage divider string in the photomultiplier which places each dynode under incremental voltages. A voltage is then applied to the isolated dynode which can vary anywhere from the voltage the isolated dynode would have had if inserted in the voltage divider string to the voltage that the immediately preceding or immediately succeeding dynode in the string has whereby the photomultiplier&#39;s gain is controlled. The applied voltage to the isolated dynode is developed electronically by a voltage to frequency converter coupled by an opto-isolator to a gain voltage divider circuit which cycles the applied voltage between two different voltage potentials tapped from the voltage divider string. The voltage to frequency converter is controlled by individual gain signals developed and stored during calibration of the camera for each photomultiplier.

This is a continuation of application Ser. No. 247,063 filed May 20,1994, now U.S. Pat. No. 5,493,610.

FIELD OF INVENTION

This invention relates generally to a gain control circuit forphotomultipliers used in gamma cameras.

The invention is particularly applicable to and will be described withspecific reference to a gain control arrangement for a gamma cameraresulting in improved scintigrams. However, those skilled in the artwill recognize that the invention may have broader application as a gaincontrol for any photomultiplier utilized for photon counting.

INCORPORATION BY REFERENCE

The following United States patents are incorporated by reference hereinso that details, concepts and structures known to those skilled in theart need not be repeated herein:

    ______________________________________                                        U.S. Pat. No.                                                                            Title                                                              ______________________________________                                        3,714,441  Photomultiplier Gain Control Circuit                               4,091,287  Scanning Radiology with Initial Scan for                                      Adjusting System so that Detector Means                                       Operates Within its Preferred Range                                4,808,826  Smooth Dot Density Spatial Distortion                                         Correction in Photon Imaging Devices                               4,866,615  Scintillation Camera Compensation for                                         Shifting the Center Channel of the Energy                                     Spectrum Due to Photomultiplier Gain                                          Change                                                             ______________________________________                                    

None of the patents incorporated by reference form a part of the presentinvention.

BACKGROUND OF THE INVENTION

All gamma cameras include a lead collimator through which gamma rays arepassed so that only those rays parallel to the slits in the collimatorstrike a scintillation crystal. The light of individual scintillationsemanating from the scintillation crystal is not collimated but spreadsout and travels through light tubes or fiberoptics to strike a pluralityof photomultipliers which are usually arranged in a hexagonal array. Thelocation of the point of scintillation origin is then obtained byalgorithms based on a weighted average which analyzes all the individualsignals from the photomultipliers. Specifically, the electrons orsignals produced in the photomultipliers in response to the photonsdetected are essentially counted in pulses. Each pulse is formed into anintensity signal, z, which is correlated to the energy of the sensedphoton(s) and a position signal, x,y, which is correlated to the pointwhere the signal originated. The x-y and intensity signals are thencorrected for energy, linearity and uniformity and, after a sufficientnumber of counts have been obtained, form specific pixels on a CRT(cathode ray tube) screen where the image of the radiated organ isproduced.

Photomultipliers for gamma cameras are supplied with gain controlboards. Typically, the photomultipliers are matched in the array to haveabout the same gain. Generally speaking, differences in gain betweenphotomultipliers are accounted for by computer weighting of eachphotomultiplier during camera calibration vis-a-vis look-up tablesstored in computer memory. In some instances the photomultipliers arepurchased with an adjustable gain control effected by a potentiometerwhich is manually set or adjusted to a desired gain so that all thephotomultipliers in the camera can have approximately the same gain.

More specifically, photomultipliers are typically selected with gaincharacteristics which are sized to produce substantially linear outputsfor the photon energy levels which are detected. The photomultipliersare then calibrated by exposing the camera to a uniform known radiationsource. See for example U.S. Pat. Nos. 4,866,615, 4,091,287 and4,808,826. Typically a pinhole or slotted aperture lead mask ispositioned in front of a reference radiation beam which produces auniform radiation signal for all the photomultipliers. Manufacturingvariations between photomultipliers cause variations in the photoanodeoutput signal or variations in gain to occur among the photomultipliers.Heretofore, the industry has "adjusted" the variations by simplycomparing the signals from all the photomultipliers and factoring them,mathematically, so that each photomultiplier's signal are mathematicallyadjusted to have the same gain as that photomultiplier which is theleast sensitive or has the smallest gain in the photomultiplier array.The "gain" values for each photomultiplier are then stored in a"look-up" table within the camera's computer. When the isotope for whichthe camera has been calibrated is used in a patient, the "look-up" tablefactors each photomultiplier signal by the value stored in the table. Itis appreciated of course that there is a separate look-up table for eachisotope which the camera senses, and it is not uncommon for there to beas many as 27 or so look-up tables corresponding to the differentradioactive isotopes used in the medical field today.

It is, or should be, obvious that the greater the gain signal differencebetween photomultipliers for any given isotope, the more significant thefactoring becomes leading to the possibility of error. It also should berecognized that the more complicated the factoring becomes toextrapolate gain signals from exponential curves, a large amount ofcomputer memory is required and the time for calibration is increasedaccordingly.

Using photomultipliers with manually adjustable gain control mechanismsdoes not resolve the problem discussed above. First, while the gain foreach photomultiplier can be set to the approximate gain of one anotherwhen the camera is calibrated for one specific isotope, the adjustmentis only approximate. It can never be precise. Secondly, while anapproximate adjustment can be made for one isotope the fact that any onespecific photomultiplier may not be linear for another isotope means thedifferences have to still be accounted for by automatically factoringthe signals during calibration before the signals are stored in thelook-up table. Thus, while manual gain control adjustments are helpfulin that at least there is an attempt to remove any significant disparitybetween photomultipliers, they are not a solution to the problem.

Apart from variations in gain between photomultipliers which aresupposedly resolved during calibration, calibration is also used toremove the noise inherently present in the photomultiplier. That is withthe photomultiplier off, a signal is still produced at the anode whichis termed noise. This signal is measured, stored and subtracted from theoutput signal produced during operation of the photomultiplier. Varioustechniques have been used to shut off the photomultiplier such as bytying the dynodes in the divider voltage resistor string together whichmaintains voltage in the voltage divider resistor string while shuttingoff the photomultiplier.

As is well known, the gain characteristics of the photomultiplierschange in time producing errors. See for example U.S. Pat. Nos.4,866,613 and 4,808,826 where the problem is discussed at some length.The solution followed by the industry as a whole has been to modify thecompensating tables to compensate for the photomultiplier gain changeand in this manner produce an accurate picture. However, because thecompensation table itself is not linear, it is quite possible that anymodification thereof as well as the initial table, can in turn produceerror.

Insofar as controlling the gain of a photomultiplier, reference shouldbe had to Photomultiplier Handbook, by Burle Industries, Inc,copyrighted 1980 (Printed 1989) Chapter 5 which discusses various gaincircuits. The Handbook notes that while a separate voltage supply couldbe used for each dynode in the photomultiplier, a resistive voltagedivider circuit is generally utilized for the dynodes and that the firstdynode region should have a high cathode-to-first-dynode voltage forcertain applications. Gain is then usually controlled by adjusting theoverall or line voltage inputted to the voltage divider string. To avoidspace charge effect, current in the voltage divider circuit should beten times the anode current which, in the case of high gain gammacameras, result in high power dissipations causing resistor heat which,in turn, affect the linearity of the photomultiplier's output signal.The Handbook also notes that certain dynodes can be tied so that thephotomultiplier need not operate with all its stages and that where theoverall voltage is not to be changed, it is possible to set the gain bysetting the voltage of a single dynode. The frequently employed methodto establish gain is to simply vary the overall voltage. With respect toautomatic gain control circuits, reference can be had to U.S. Pat. No.3,714,441 which utilizes a comparator circuit to adjust the line voltageto maintain the photomultiplier's gain at a desired value.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the invention, to provide animproved gain control for the photomultipliers used in a gamma camera toproduce accurate and clear scintillation images.

This object along with other features of the invention is achieved in ascintillation gamma camera which has a plurality of photomultipliers.Each photomultiplier has within a vacuum enclosed space a photocathode,an anode and a plurality of dynodes spaced incrementally between thephotocathode and the anode and numbered sequentially from thephotocathode as d₁, d₂, d₃, etc. with any specific dynode designated asd_(n). A line voltage potential arrangement which includes a resistorvoltage divider string and a power supply applies a D.C. voltage to eachdynode at incremental potentials corresponding to the numbering of thedynodes so that the voltage potential of one dynode is less than thevoltage potential at the next numbered dynode etc. A conventionalmechanism converts the electrical signals generated by thephotomultipliers into a scintillation image. A gain mechanism isprovided for electronically establishing the gain of eachphotomultiplier to be equal to one another in accordance with theradiation of a predetermined test beam. In accordance with theinvention, the electronic gain mechanism includes an isolatorarrangement for isolating the specific d_(n) dynode from the voltagedivider resistor string and a circuit is provided to connect thespecific d_(n) dynode to a DC voltage potential which is set anywherebetween the voltage the specific d_(n) dynode would have, had thespecific d_(n) dynode been inserted in the string and the lower voltagepotential applied by the string to the dynode d_(n-1) which isimmediately adjacent to the position the specific dynode d_(n) wouldhave had in the string whereby the gain of the photomultiplier isdetermined by the voltage potential applied to the specific d_(n)dynode.

In accordance with a more specific feature of the invention, theisolating arrangement includes a first tap line extending from thestring to ground at a position whereat the tap line has a firstreference string voltage, d_(n-REF), which would have been applied tothe specific d_(n) dynode had the specific d_(n) dynode been insertedinto the string and a second tap line extending from the immediatelypreceding d_(n-1) dynode and having the voltage potential of the stringapplied to the d_(n-1) dynode. The connecting circuit includes a secondresistive voltage divider for variably dividing the voltage between thefirst and second tap lines-which includes a control node connected tothe specific d_(n) dynode, a first resistor between the control node andthe first tap line, a second resistor between the control node and thesecond tap line; a switch between the control node and the second lineand a mechanism to control the opening and closing of the switch inaccordance with a stored gain signal voltage whereby the voltagepotential of the specific dynode d_(n) is set at a voltage between thefirst and second tap lines. In this manner a carefully controlledvoltage from the photomultiplier's resistor voltage divider string istapped to accurately set the gain of the photomultiplier without anyundue variations in voltage which might otherwise occur while allowingthe line voltage to the photomultiplier to remain set at a constantoptimum value for the sensed radiation.

In accordance with a still further specific and important feature of theinvention, the switch mechanism includes an opto-isolator, and theswitch control arrangement includes a voltage to frequency converterwith a gain signal voltage correlated to the voltage imposed on thespecific dynode d_(n) inputted to the frequency converter so that thegain arrangement avoids generating excessive heat from the high dynodevoltage potentials which could otherwise affect the linearity of thephotomultiplier's response. R-C filters are used to smooth the switchedvoltages to produce a constant voltage applied to the specific d_(n)dynode.

It is another object of the invention to provide an improved gaincontrol for a photomultiplier and like devices which accuratelyestablishes the gain of the photomultiplier in accordance with a presetgain control signal.

Yet another specific object of the invention is to provide an electronicgain control arrangement for a gamma camera which assures the integrityof the voltage divider resistor string in the photomultiplier and thentaps the string to produce a precise, consistent gain voltage for eachphotomultiplier within the camera.

Still another general object of the invention is to provide a gaincontrol mechanism for any photomultiplier used to convert photons intoelectrical impulses which can accurately be adjusted within the range of100% down to about 50% of the voltage output for any givenphotomultiplier.

A still further object of the invention is to use an electronic gaincontrol circuit for a photomultiplier which minimizes heat generatedduring operation of photomultiplier thus improving the linearity of thephotomultiplier response.

A still further object of the invention is to provide a simple shut offcircuit for a photomultiplier which can be operated to shut off selectedphotomultipliers in a gamma camera array.

It is another general object of the invention to provide an electronicgain control circuit for a photomultiplier or an array ofphotomultipliers whether or not said photomultipliers are used in agamma camera or other photon detecting device.

Still another more specific and important feature of the invention is toprovide a gain control for a photomultiplier which establishes the gainof the photomultiplier while insuring the integrity of the dynodevoltage divider string to minimize voltage variations between dynodes orstages.

These and other objects of the invention will become apparent to thoseskilled in the art upon reading and understanding the DetailedDescription of the invention set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangementsof parts, a preferred embodiment of which will be described in detailand illustrated in the accompanying drawings which form a part hereofand wherein:

FIG. 1 is a general schematic view of a gamma camera including the basicsignal information components shown in block form.

FIG. 2 is a graph of an electrical pulse produced by the photomultiplieras a result of a gamma ray producing a burst of scintillated light;

FIG. 3 is a schematic representation of the circuit boards attached tothe photomultiplier showing the connections therebetween;

FIG. 4 is an electrical circuit schematic of the voltage divider boardshown in FIG. 3;

FIG. 5 is an electrical circuit schematic of the gain control boardshown in FIG. 3;

FIG. 5a is a redrawn portion of the voltage divider circuit used in thegain control board shown in FIG. 5;

FIG. 6 is an electrical circuit schematic of the preamp board shown inFIG. 3;

FIG. 7 is a graph of the voltage potential range of a given dynode;

FIG. 8a is a graph of the voltage frequency output from a voltage tofrequency converter used in the gain control circuit shown in FIG. 5;and

FIG. 8b is a graph of the voltage frequency shown in FIG. 8a but in afiltered state;

FIG. 8c is a graph of the voltage frequency shown in FIGS. 8a and 8b ina final filtered, steady-state condition.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein the showings are for the purposeof illustrating a preferred embodiment of the invention only and not forthe purpose of limiting same, there is shown in FIG. 1, in generalschematic form a nuclear camera 10 of the Anger type. Camera 10 includesa lead collimator 11 for catching and directing certain parallelincident rays of radiation 12 onto a scintillation crystal 13. As isknown in the art, radiation rays are emitted from the organs of apatient as a result of a radioactive isotope such as iodine ingested bythe patient. Scintillation crystal 13 in turn produces as a result ofabsorbing a ray of radiation 12 a burst of light which is directed bylight tubes 14 onto the photocathode of a plurality of photomultipliertubes 16 hereinafter referred to as photomultiplier or PET.

In camera 10 of the preferred embodiment scintillation crystal 13 isrectangular (about 20"× 26") and photomultipliers 16 have hexagonal faceplates and are packed in a hexagonal array of 58 tubes. Photomultipliers16 emit photoelectrons from the scintillated burst of light which arecreated by the photocathode and are directed by an appropriate electricfield to an electrode or dynode which in turn emits a greater number ofsecondary electrons directed to the next dynode etc., until a high gainof electrons are collected by the anode which provides a signal outputcurrent correlated to the incident ray of radiation. "Gain" as used inthis specification will have the meaning ascribed it as set forth in thePhotomultiplier Handbook, namely, "the ratio of (1) the output signalcurrent to (2) the photoelectric signal current from the photocathode".The radiation signal produced by photomultipliers 16 is amplified anddirected to an analog to digital converter 21 where it is digitized andthe digitized radiation signal is then corrected for i) energydistortion by an energy distortion circuit 22, ii) linearity through alinearity circuit 24 and iii) uniformity through a uniformity circuit25. The reformed radiation signal is then inputted to a cathode ray tube26 where each signal produces pixels of various shades which make up ascintigram. As thus far described, camera 10 is conventional.

As noted in the Background Discussion, the prior art cameras which donot have an adjustable gain for each photomultiplier factor thephotomultiplier tube's output signal vis-a-vis look up tables stored inthe computer's memory and generated during calibration of the camera. Intheory, the factoring can be done before radiation signal is digitized,or after it is digitized or the algorithms in the correcting circuits22, 24 and/or 25 can account for the factoring since the correctioncircuits are essentially factoring the radiation signal in accordancewith their own tables. On those cameras where photomultipliers 16 aresupplied with an adjustable gain control, a technician manually adjustseach photomultiplier so that the gain of each photomultiplierapproximately equals the lowest gain obtained by the least responsivephotomultiplier for one specific test radiation beam. Since theadjustment is manual and other isotopes produce differentphotomultiplier gains which will inevitably not produce the same gainsfor all the photomultipliers, look-up factoring tables still exist andthe process continues in the same manner as it does for photomultipliers16 with a fixed gain. In accordance with the present invention gainadjustment of each photomultiplier 16 is accomplished electronically foreach radioactive isotope sensed by camera 10 and the adjustment is suchthat all photomultipliers have the same gain although that gain may verywell be different for each isotope. The gain adjustments are effected byindividual PMT gain signals stored in the camera's computer when camera10 is calibrated. Calibration and recalibration, when necessary, is anautomatic event once camera 10 is positioned within the propercalibrating grids and subjected to test radiation beams as describedhereafter.

The electronic circuits which adjust the gain of each of the 58photomultipliers 16 are shown in FIGS. 3-6. As best shown in FIG. 3, thecircuits are contained on three printed circuit boards which are avoltage driver or divider circuit board 30 (FIG. 4), a gain controlcircuit board 32 (FIG. 5), and a preamp circuit board 34 (FIG. 6).Circuit boards 30, 32, and 34 are mounted to the rear of photomultiplier16 with the divider board making contact with the photocathode, anodeand dynodes of photomultiplier tube 16. Power and interconnectionsbetween circuit boards 30, 32, and 34 are through several connectors.

Specifically, there are two external connectors 37, 38 mounted to preampcircuit board 34. External connector 37 is a 4 pin connector whichconnects to a 4 pin preamp board connector 39 on preamp circuit board 34which in turn connects to a 4 pin board connector 40 on voltage dividercircuit board 30. All connector pins are shown in the drawings numberedsequentially from reference numeral 1 in FIGS. 4-6 and for 4 pinconnectors 37, 39, 40 the pin designations are as follows:

    ______________________________________                                        Connectors                                                                    37, 39, 40                                                                    Pin No.            Function                                                   ______________________________________                                        1                  +HV Input                                                                     (from HV Power                                                                supply)                                                    2                  NC (no                                                                        connection)                                                3                  NC (no                                                                        connection)                                                4                  Gnd                                                        ______________________________________                                    

A ten pin external connector 38 is also provided preamp circuit boardwith pin designations as follows:

    ______________________________________                                        Connector 38                                                                  Pin No.            Function                                                   ______________________________________                                        1                  +15V                                                       2                  PMT Gain Volt                                                                 (from computer)                                            3                  Signal Gnd                                                 4                  PMT Off                                                    5                  PMT Signal Out                                             6                  Signal Gnd                                                 7                  Signal Gnd                                                 8                  Signal Gnd                                                 9                  -15V                                                       10                 Signal Gnd                                                 ______________________________________                                    

In addition, preamp circuit board 34 has an 8 pin board connector 42which connects to an 8 pin board connector 43 on gain control circuitboard 32. Connector pins are numbered and designated in function asfollows:

    ______________________________________                                        Connector 42, 43                                                              Pin No.             Function                                                  ______________________________________                                        1                   +15V                                                      2                   PMT Gain Volt                                                                 (from computer)                                           3                   Signal Gnd                                                4                   PMT Off                                                   5                   PMT PA (Photo                                                                 Anode) out                                                6                   Signal Gnd                                                7                   Signal Gnd                                                8                   Signal Gnd                                                ______________________________________                                    

Also, gain control circuit board 32 has an 8 pin board connector 45which connects to an 8 pin board connector 46 on voltage divider board30, Connector pins are numbered and designated in function as follows:

    ______________________________________                                        Connector 45, 46                                                              Pin No.             Function                                                  ______________________________________                                        1                   +HV Gnd                                                   2                   PMT PA out                                                3                   NC (no connec-                                                                tion)                                                     4                   +HV Gnd                                                   5                   Dynode N-1 Ref                                            6                   Dynode N-2 Ref                                            7                   Dynode N Ref                                              8                   Dynode N                                                  ______________________________________                                    

Referring first to the resistor voltage divider circuit shown in FIG. 4,photomultiplier 16 has its photocathode 50 connected to ground 51 andits anode 52 at positive line voltage which is outputted to pin 2 ofconnector 46. Photomultiplier 16 has ten stages or dynodes (excludingfocus electrode 53) which are labelled in FIG. 4 as "dynode 1" through"dynode 10". A venetian blind photomultiplier is schematicallyillustrated and is, in fact, used in the preferred embodiment. However,other photomultiplier designs can be used and various numbers of dynodesor stages can be used.

As noted above, high voltage to photomultiplier tube 16 is inputted atpin 1 of internal connector 40. As will be explained below, the highvoltage is constant but its voltage is set by the camera's computer 28(FIG. 6) to be at an optimum voltage for the specific radioactiveisotope which camera 10 is sensing. It is also to be understood that onepower supply (shown schematically in FIG. 6 as reference numeral 29)furnishes the power at a preset DC voltage for all 58 photomultipliers16 used in camera 10. This permits conventional techniques to be used toclosely regulate the input voltage to assure steady state high voltagesfor all PMT's. The input voltage which determines the overall gain orlimits of the maximum gain of photomultiplier 16 can be preset anywherefrom about 600 to 1500 volts D.C. depending on the isotope being sensed.

The inputted high voltage is at a high point at node 57 of FIG. 4 and isdivided somewhat equally among the dynodes or stages by a resistive orresistor voltage divider string placed across the high voltage source,i.e., node 57 and ground 51 which string is generally contained withinthe dot-dash envelope designated "A" in FIG. 4. That is a plurality ofequally sized resistors 54a through 541 connected in series divides thehigh voltage equally among the ten dynodes. (All electrical circuitcomponents shown in the drawings which are identical will have the samereference numeral assigned to the component. Any specific componentdiscussed in the specification will be distinguished from otheridentical circuit components by the letter subscript following thereference numeral. Thus, the 12 resistors identifying the resistorvoltage divider string A for the 10 stage (plus focus electrode 53)photomultiplier 16 are identified by subscripts "a" through "1"following reference numeral 54.) For ease of discussion all resistors 54are equal, but are not required to be equal in a given application.Thus, assuming an input voltage of 1500 volts the voltage potentialbetween adjacent dynode varies incrementally by 125 volts. Specificallyfocus electrode 53 is at 125 volts, dynode 1 is at 250 volts, dynode 3is at 375 volts, etc. until dynode 10 is at 1375 volts. In practice, theresistors at focus electrode 53 and dynode 1 are at higher resistancesthan the other resistors so that a greater voltage potential exists atfocus electrode 53 and the first dynode. The resistive voltage divideris conventional as is the high voltage potential for the cathode andfirst dynode region and is discussed at pages 81-83 of thePhotomultiplier Handbook (incorporated by reference herein).

Also, divider circuit 30 uses charge storage capacitors 56 connected inseries with one another. Because the radiation detected by camera 10emits pulses of light, the output signal produced at anode 52 ofphotomultiplier 16 is in the form of pulses. The resistance of thevoltage-divider network discussed above is based on the average anodecurrent. When the average anode current is much less than the peak pulsecurrent, the dynode potentials can be maintained at a nearly constantvalue during the pulse duration by use of charge-storage capacitors. ThePhotomultiplier Handbook suggests that the capacitors be located at thedynode socket and discloses series or parallel examples for allphotomultiplier stages with capacitance increasing as the stagesapproach the anode. In the FIG. 4 circuit of the present invention,there are only five equal fixed charge capacitors 56a through 56e fordynodes 10 through 6 because, as shown in the table below, the number ofsecondary electrons emitted (and thus the PMT current) increasessignificantly only for the last photomultiplier stages.

There are four NPN transistors 58a, b, c, and d in series with oneanother and three PNP transistors 59a, b, and c in series with oneanother which are connected together in a transistor string to groundshown generally within a dot-dash envelope designated "B" in FIG. 4.Each NPN transistor 58 has, respectively, a diode 60a, b, c and d,associated therewith and each PNP transistor 59 has, respectively, adiode 61a, b and c associated therewith. By connecting each diode to thetransistor base as shown, a forward voltage is set by the diode toprotect transistor 58, 59 from turn-on/turn-off transient voltages orcurrent while also functioning to protect the transistor in the event ofa PMT failure. NPN transistor 58a's emitter connects at node 63 todynode 10. NPN transistor 58b connects at node 64 to dynode 9. NPNtransistor 58c's base connects at node 65 to dynode 7. NPN transistor58d's base connects to node 75. PNP transistor 59a's base connects atnode 66 to dynode 4. PNP transistor 59b's base connects at node 67 todynode 3 and PNP transistor 59c's base connects at node 68 to dynode 1.

As is well known, the dynode current increases significantly at the lastdynode stages. For example, the dynode current distribution for a 100 μaphotoanode current is typically as follows:

    ______________________________________                                                      Dynode        Drop                                              Dynode        Current       Across 1 mΩ                                 ______________________________________                                        1:            6.67 × 10.sup.-4 μa                                                                6.67 × 10.sup.-4 v                          2:            .002 μa    2 mv                                              3:            .007 μa    7 mv                                              4:            .024 μa    24 mv                                             5:            .028 μa    78 mv                                             6:            .258 μa    258 mv                                            7:            0.85 μa    850 mv                                            8:            2.8 μa     2.8 v                                             9:            9.221 μa   9.22 v                                            10:           30.366 μa  30.36 v                                           Photoanode:   100 μa     100.00 v                                          ______________________________________                                    

Further, the Photomultiplier Handbook suggests for linear responsepurposes that the voltage divider current be 10 times the photoanodecurrent. Following the Handbook recommendations, and given the dynodecurrent levels, high heat from current flow through the resistors willbe generated even if a tapered divider network (varying resistances) isused. By using the emitter-follower characteristic of transistors 58 itis possible to provide a power supply requiring much less dividercurrent and thus less heat. As is well known, heat adversely affects thelinearity of the photomultiplier's response. In the circuit shown inFIG. 4, as the dynode current increases, the added current is divertedfrom transistors 58 rather than from the resistor-capacitor dividerstring. This results in significantly improved voltage regulationbetween the dynodes which remains constant and at a lesser current drawthan the 10 to 1 suggestion of the Photomultiplier Handbook. Thus, aconstant, non-varying voltage potential between dynodes with less heatfrom current flow is obtained by the FIG. 4 circuit. Reference can behad to FIG. 96 in the Photomultiplier Handbook for a different activedivider network also using transistors to divert the added dynodecurrent from the resistor-divider string.

In the present invention and from a study of the PMT dynode currenttable shown above, only the dynode current from the last and second lastdynodes, dynodes 10 and 9, are significant. Thus, the heat resultingfrom the added current is dissipated by transistors 58a and 58b and thecurrent is high enough for additional charge-storage capacitors 56f and56g to be used to smooth the pulses. Thus, transistors 58a and 58bdissipate heat. To completely isolate the added PMT dynode current fromthe resistor voltage divider string A, (and unlike the FIG. 96Photomultiplier Handbook circuit), a series transistor string Bconnected to ground 51 is constructed. Since the dynode current isdropping, NPN transistors 58a-d are used for the higher voltage stagesand connect collector to emitter to "push" the current and PNPtransistors 59a-c are used for the lower voltage stages and connectemitter to collector to "pull" the current to ground 51. A resistor 80is used as a "keeper" to maintain current flow from the NPN transistorsto the PNP transistors to maintain the series connection of thetransistor string B.

Also shown in FIG. 4 is resistor 69 and capacitor 70 which act as an RCfilter for the high voltage to remove spikes or glitches. The anodeoutput signal from photomultiplier 16 is outputted on line 82 throughcoupling capacitor 83 to pin 2 of connector 46.

At pin 8 of connector 46, a dynode "N" voltage potential is directlyinputted to dynode 5 on line 73. In the preferred embodiment dynode 5 isthe specific, string isolated dynode, d_(n), which has a preset voltagethat determines the gain of photomultiplier 16. At first pin 5 ofconnector 46 a dynode "N reference" voltage connects on tap line 74 tonode 75 on the resistor voltage divider string between resistors 54f and54g which would be the node in the resistor voltage divider string Awhere dynode 5 would have connected had line 73 not existed. As will beshown in the description of gain circuit 32, tap line 74 insures thatresistor divider voltage potential distribution on string A remainsintact while the voltage potential at dynode 5 is determined solely bythe voltage of line 73. In this manner, dynode d_(n) is isolated fromresistor voltage divider string A. In addition, a second tap line 77from pin 5 of connector 46 has a voltage potential shown as dynode "N-1reference" which in turn is connected at node 66 to dynode 4 through PNPtransistor 59a. Second tap line 77 is thus at the voltage potentialapplied to the immediately adjacent dynode d_(n-1) closer tophotocathode by resistor voltage divider string A. Also, a third tapline 78 from pin 6 of connector 46 has a voltage potential referred toas dynode "N-2 reference" applied to dynode 3 at node 67 through PNPtransistor 59b. Third tap line 78 is thus at the voltage potentialapplied by resistor voltage divider string A to dynode d_(n-2) which isimmediately adjacent dynode d_(n-1) on the side towards photocathode 50.Resistor 80, as noted, interconnects dynode d_(n-REF) tap line 74 withdynode d_(n-1-REF) tap line 77 and acts as a current keeper maintainingPNP transistors 59 in series with NPN transistors 58.

It is important to note that the voltages on tap lines 74, 77 and 78 aredetermined solely by resistor voltage divider string A becausetransistor string B has removed any adverse influence attributed to thePMT dynode current. Further, the high voltage to resistor voltagedivider string A is constant. Thus, an accurate, uniform basis isestablished to regulate the voltage on dynode d_(n) which helps removeany individual PMT dynode d_(n) (dynode 5 in the preferred embodiment)voltage variation which can result in influencing the gain set for eachphotomultiplier 16.

Referring now to FIG. 6, there is shown a somewhat conventionalamplifier circuit in which the output signal 82 from anode 52 ofphotomultiplier 16 is inputted to preamp circuit board 34 through pin 5of connector 42 on line 85 and is outputted as an amplified signal online 86 to pin 5 of external connector 38 where the signal is sent to anintegrator for reformation into the x, y and signals. Thephotomultiplier anode output signal is amplified by transistor 90functioning as an inverting amplifier (shown as reference numeral 20 inFIG. 1). Because the light pulses arise and diminish over a time spanmeasured in nanoseconds, "standard" type operational amplifiers take toolong a time to recover from an input overload condition (backgroundradiation) to function satisfactorily for this application. Thus,transistor 90 is selected as a RF (radio frequency) transistor which hasa response time in nanoseconds. Apart from the selection of an RFtransistor as an amplifier for a gamma camera application, amplifiercircuit 34 is somewhat conventional.

Amplifier circuit power in is at plus 15 volts at pin 1 on line 91, andminus 15 volts at pin 9 on line 92 of external connector 38 thusproviding a dual plurality power supply. Signal ground is on line 93.Power supply filters are provided by resistor 95a and capacitors 96a and97a and similarly by capacitors 96b and 97b and resistor 95b and also byresistor 98 and capacitor 99. Diodes 100 a, b function as inputprotectors for the anode signal connected to the base of transistor 90.Because RF transistor 90 is metal shielded, the shield is connected toground on line 101. Base resistors 102, 103 form a voltage divider andare sized to properly fix the base voltage, more specifically the biascurrent or "Q" point of transistor 90. Emitter resistor 105 andcollector resistor 106 set a fixed gain to transistor 90 in proportionto the resistance value of collector resistor 106 to emitter resistor105 (i.e., the ratio of 106/105). In conventional photomultiplier ampcircuits, an adjustable gain of the photomultiplier could be effected bysubstituting a potentiometer for collector resistor 106 so that itsresistance can be varied. In accordance with the invention, theresistances are fixed since the circuits disclosed will establish afixed gain for photomultiplier 16. Finally, a NPN transistor 108 isprovided as a voltage follower having an emitter resistor 109. Signaloutput from transistor 108 passes through a coupling capacitor 110 andoutput line 86 is connected to signal ground through resistor 111.

Gain control circuit 32 is disclosed in FIG. 5. A photomultiplier gainsignal is inputted to gain control circuit 32 on line 120a through pin 2of connector 43. The gain signal is established by computer 28 whichdetermines the gain for each photomultiplier 16 during calibration. Inaccordance with the invention, gain control circuit 32 electronicallyadjusts the gain of photomultiplier 16 and also provides a control toshut off photomultiplier 16. The general principle by which this occursis discussed below.

Photomultiplier Gain Control System

The technique by which a Photomultiplier Tube's (PMT's) gain can becontrolled is achieved by varying the voltage on a given dynode withrespect to the 2 adjacent dynodes. Typically, the dynodes in a PMT aretied in a linearly increasing voltage potential (from photocathode tophotoanode) and electrons are accelerated from dynode to dynode due tothe positive voltage gradient seen from the previous dynode to the nextdynode. This invention takes one of the dynodes in the linear voltagestring and changes the voltage on a specific dynode such that theacceleration potential for the electrons coming from the previous dynodeis changed. For example, if a 10 dynode PMT has a total applied voltageof 1100 volts and the voltage is equally divided among the 10 dynodes,each dynode will have an increased voltage potential of 100 volts (e.g.dynode 1 is at 100V, dynode 2 is at 200V, etc.). To control the gain ofthe PMT tube, dynode 5 (which normally sits at 500V) is varied from 400V(dynode 4 potential) to 600V (dynode 6 potential). This is graphicallyillustrated for the gain of dynode "n" in FIG. 7. None of the otherdynode voltages are varied in this gain adjustment. Variations in gainin excess of 50% can be realized using this technique.

While in theory the voltage potential on a given dynode can vary betweenthe voltage of adjacent dynodes d_(n+1) and d_(n-1), in the preferredembodiment the gain is accomplished by varying the voltage of d_(n)between the voltage d_(n) would normally have had in the voltage-dividerstring A d_(n-REF) and the lesser voltage of the immediately precedingdynode d_(n-1). This is shown by the shaded area of the gain curve inFIG. 7, since the gain curve is symmetrical about its mid-point.Alternatively, the gain could be adjusted between d_(n-REF) and theimmediately succeeding dynode d_(n+1). In accordance with the broadconcept of the invention, the voltage potential d_(n) can vary anywherebetween the voltage which would have been applied to dynode d_(n) ifinserted in resistor voltage divider string A (d_(n-REF)) and thevoltage at any point on the resistor voltage string. Further, while thegain of photomultiplier 16 could be controlled by so varying thepotential on any given dynode, it is preferred to vary the potential ofa dynode in the middle of resistor divider voltage string A since theearly stages have few secondary electrons and the later stages have alarge number of secondary electrons thus making gain adjustmentsdifficult. This is diagrammatically illustrated by the dash linesillustrating gain for dynode 6 and the dash lines indicating gain fordynode 4. Stability is best achieved by selecting amid-point dynode suchas dynode 5 for the best overall gain control in the preferredembodiment.

PMT Gain Shutdown Control

The gain adjustment can be increased beyond 50% by taking the voltage ofthe specified dynode beyond the limits of the adjacent dynode'spotentials. If, for example, in the above scenario the potential ofdynode 5 (dynode d_(n)) was taken to 300V (dynode 3 or dynode d_(n-2)potential), the electrons are actually repelled coming off of dynode 4(dynode d_(n-1)) and effectively shut the tube off. The advantage ofusing this technique over just tying the first several dynodes togetheris that there is no buildup of electrons from previous gamma eventshitting the photocathode. Heretofore, these free electrons collect onthe photocathode and when the acceleration potential is re-applied,there is surge of current resulting from purging the previous history ofgamma events. By repelling the electrons in the series dynode string,these electrons from the previous dynode are re-absorbed and the pulseof current is absent upon reapplication of the PMT to normal operation.

More specifically in FIG. 5, the gain voltage signal 120 is inputted toa voltage-to-frequency converter chip 122 which is a NationalSemiconductor LM331 chip. The pins numbered 1 to 8 in FIG. 5 correspondto the pin numbers set forth in the technical specifications for theNational Semiconductor LM 331 chip and reference can be had to the chipspecifications for an understanding of the integrated circuits used inthe chip. Insofar as the invention is concerned, a square wave output isapplied to pin 3 of chip 122 onto line 123 with the relative duration orthe time the voltage is on and off determined by the voltage of PMT gainsignal voltage on line 120 which is set by computer 28. The time of theon cycle is set by RC pin 5 on line 132. Insofar as the external circuitconnections to chip 122 are concerned, gain signal 120a is filtered byresistor 125 and capacitor 126 which is connected to signal ground 127and gain signal 120b can vary anywhere from 0.0 volts for maximum gainto 2.5 volts for minimum gain. Supply voltage 128 of 15 volts from pin iof connector 43 is filtered through capacitor 129 (which in turn isconnected to signal ground 127) and supply voltage 128 is also connectedto resistor 130. Control voltage on line 132 into pin 5 of chip 122connects through resistor 133 to supply voltage 128 and to signal ground127 through capacitor 135. Threshold current on line 136 (pin 6) andoutput current on line 137 interconnect at node 138 to resistor 139 inparallel with capacitor 140, both connected to signal ground 127 andfunctioning to trigger the timing cycle whereat capacitor 140 is chargedfrom 137 (pin 1 of chip 122) and discharges itself through resistor 139.Finally, reference current on line 141 (pin 2) (which programs I_(out)from pin 1 of 137) connects to signal ground 127 through resistor 142and chip ground (pin 4) connects to signal ground 127.

As shown in FIG. 5 frequency output (pin 3) from chip 122 connects to anopto-isolator 145 which is basically an LED switch in turn controlling again resistor voltage divider string shown generally by the dot-dashenvelope designated "C" in FIG. 5 and redrawn in FIG. 5a for discussionpurposes. Conceptually, when the LED doesn't light, the switch is off,and the voltage dynode_(n-REF) from resistor voltage divider string A isinputted to dynode_(n). This is the same voltage potential which wouldhave been applied to dynode_(n) had dynode_(n) been inserted intoresistor voltage divider string A. When the diode lights the voltagepotential from dynode_(n-1) from resistor divider string A is applied tothe gain resistor voltage divider string C. The voltage applied todynode_(n) at this time is a filtered ratio (set by resistors 147 and152 and easily calculated by those skilled in the art) and ends up beingbetween dynode_(n-REF) and dynode_(n-1REF). Thus, in the preferredembodiment the limits of the gain control is set by the ratio ofresistors 147 and 152. In theory, if more of the photomultiplier's gainhad to be controlled, the ratio of resistor 147 to 152 would be changed.If necessary, a different preceding dynode potential voltage could betapped, such as d_(n-2), and the switch controlled to input a lowervoltage potential to dynode_(n). Thus, the frequency of opening andclosing of switch or opto-isolator 145 (which, in turn, is set by PMTgain voltage on line 120a,b) determines the voltage applied todynode_(n) which can control the gain of photomultiplier 16. R-C filtersare then employed in the circuit to develop a steady state gain control.

This is graphically shown in FIGS. 8a, 8b and 8c. More specifically,FIG. 8 shows the unfiltered voltage with the switch or opto-isolator 145in its "on" and "off" position. When "off" the voltage to dynode_(n) isdynode_(n-REF) voltage as discussed above and is indicated by line 114.When "on" the voltage to dynode_(n) is the ratio of voltages set byresistor 149 and 152, being between the voltages of dynode d_(n-REF) anddynode_(n-1REF) as discussed above and as indicated by line 115. FIG. 8bshows the voltage after it passes through the first filter and FIG. 8cshows the voltage after passing through the second R-C filter. Where thedynode_(n) voltage shown by line 116 in FIGS. 8b and 8c falls betweenthe limits of lines 114 and 115 is then a function of the frequency ofthe original voltage shown in FIG. 8.

Those skilled in the art will recognize that a relatively small voltage,15V, is used to actuate chip 122 and opto-isolator 145 which contains atransistor for switching the relatively large dynode voltages which areabout 500-600 volts. Besides cost considerations, use of a low voltagefrequency converter coupled with an opto-isolator avoids the heat thatwould otherwise be generated by alternative circuits which could bedesigned to generate dynode_(n) voltages pursuant to a preset PMT gainsignal.

Referring now to FIGS. 5 and 5a dynode_(n-REF) voltage is inputted ontap line 151 and dynode_(n-1REF) voltage is inputted on tap line 161.Both tap lines 151 and 161 are connected, respectively, to capacitors170a and 170b which, in turn, are connected to ground to conventionallydecouple dynode_(n-REF) and dynode d_(n-1-REF) voltages from supplyvariations. Dynode d_(n) on line 154 connects to a control node 149 ongain voltage divider string C. A first resistor 152 connects on line 153to control node 149 and similarly a second resistor 147 connects on line150 to control node 149. Second line 150 also contains switch oropto-isolator 145. When switch 145 is open current flows from first tapline at dynode d_(n-REF) voltage through first resistor 152 past controlnode 149. It is filtered through a charge storage capacitor 148 andamplified by a NPN transistor 155 after which the current passes througha third resistor 159 and is filtered a second time by a charge capacitor160 whereat the voltage is outputted to dynode d_(n). When switch oropto-isolator 145 is "on" current from second tap line 161 at dynoded_(n-1-REF) voltage passes through second resistor 147 on line 150. Asdiscussed above, the voltage potentials are summed and divided atcontrol node and inputted to dynode d_(n). A fourth resistor 164 isadded to the circuit to function as a keeper resistor so that the outputfrom node 154 tracks to control node 149.

To shut off photomultiplier 16, a second opto-isolator 175 connects athird tap line 177 having a voltage at dynode d_(n-2-REF) from resistorvoltage divider string A. The transistor within second opto-isolator 175switches current to line 179 where it is filtered by capacitor 160 fromthird tap line 177 to dynode d_(n) output line 154. The LED withinsecond opto-isolator 175 biases the transistor when a computer generatedPMT off signal from Pin 4 of connector 43 is generated on line 178.

Computer 28 also develops for each photomultiplier the PMT shut-offsignal which is sent to gain control board 32 to shut off any selectedphotomultiplier 16. By selecting any specific photomultiplier to be shutoff, it is possible to better calibrate a given photomultiplier in thatthe surrounding photomultipliers can be left shut off while thecalibrated photomultiplier is turned on. In this way, the surroundingphotomultipliers will not contribute to the noise of the turned onphotomultiplier. Thus a better way to isolate the signal is presentedbecause a better low noise signal is produced. Also, during operation ofcamera 10 it now becomes possible to shut off photomultipliers which areremoved from the scintillation event.

The invention has been described with reference to a preferredembodiment. Alternations and modifications may become apparent to thoseskilled in the art upon reading and understanding the detaileddisclosure of the invention as described herein. For example, theinvention has been explained with the cathode at ground and the anode orthe last dynode at high voltage. The cathode could be placed at negativehigh voltage and the anode or the last dynode at ground. It is intendedto include all such modifications and alterations insofar as they comewithin the scope of the present invention.

    ______________________________________                                        CIRCUIT COMPONENTS                                                            Reference   Item                                                              Numeral     (Description) Part Number                                         ______________________________________                                        16          Photomultiplier                                                                             XP2442BM9612Y                                                                 (Phillips)                                          54          resistor      1 M ohm                                             56          capacitor     .01 μF, 200V                                     58          NPN transistor                                                                              2N6517 Motorola                                     59          PNP transistor                                                                              2N6520 Motorola                                     60          diode         1N4148                                              61          diode         1N4148                                              69          resistor      100K                                                70          capacitor     .047 μF, 2KV                                     71          resistor      31K                                                 72          resistor      3.3K                                                80          resistor      2.2M                                                82          capacitor     .022 μF, 2KV                                     90          RF transistor 2N4958 Motorola                                     95          resistor      470                                                 96          capacitor     .1 μF                                            97          capacitor     47 μF                                            100         diode         1N4148                                              102         resistor      6.49K                                               103         resistor      23.2K                                               105         resistor      330K                                                106         resistor      3.3K                                                108         NPN transistor                                                                              2N3904 Motorola                                     109         resistor      6.2K                                                110         capacitor     1.0 μF                                           111         resistor      10K                                                 122         voltage to    LM 331 Nation-                                                  frequency con-                                                                              al Semi Con-                                                    necter        ductor                                              125         resistor      10K                                                 126         capacitor     .1 μF                                            129         capacitor     10 μF                                            130         resistor      2.2K                                                133         resistor      90.9K                                               135         capacitor     .001 μF                                          139         resistor      100K                                                140         capacitor     .068 μF                                          142         resister      75K                                                 145         opto-isolator H11D1 Motorola                                      146         resistor      47K                                                 147         resistor      220K                                                148         capacitor     0.68 μF, 200V                                    152         resistor      220K                                                155         NPN transistor                                                                              2N6517 Motorola                                     156         diode                                                             159         resistor      2.2M                                                162         capacitor     .068 μF, 1KV                                     163         resistor      2.2M                                                170         capacitor     .01 μF, 1KV                                      175         opto-isolator H11D1 Molorola                                      176         resistor      100K                                                ______________________________________                                    

Having thus described the invention it is claimed:
 1. A gain control fora plurality of photomultipliers used in a gamma camera having an anode,a photocathode and a plurality of dynodes therebetween, said dynodesnumbered sequentially from said photocathode to said anode with thefirst dynode adjacent said photocathode designated d_(n) and any onespecific dynode designated d_(n) ;said photocathode connected to ground;a voltage divider resistor string extending from said anode to saidphotocathode, said string including a plurality of string resistors inseries with one another extending from said anode to said photocathodeand ground, each of said string resistors adjacent a dynode and numberedsequentially from said photocathode to said anode with the first one ofsaid string resistors numbered r₁ and the resistor adjacent saidspecific dynode d_(n) designated r_(n) each one of said string resistorsseparated from one another by a node; a source of constant DC voltageinputted to said string adjacent said anode; all of said dynodesconnected to said string through a node adjacent each one's respectiveresistor except for said specific dynode d_(n) which is isolated fromsaid string, a first tap line connected to the node in said stringadjacent said specific resistor r_(n) whereby the voltage potential fromsaid string associated with said dynode d_(n) is applied to said firsttap line thereby maintaining the integrity of said resistor string; andgain means electronically connected to said dynode d_(n) and utilizingsaid voltage potential on said first tap line to apply a preset gainvoltage potential to said dynode d_(n) which can vary anywhere from saidfirst tap line voltage potential to the string voltage potential at saiddynode immediately adjacent said specific dynode d_(n) on the dynodeside closer to said photocathode and designated d_(n-1) whereby saidpreset gain voltage establishes the gain of said photomultiplier.